Vinylidene complexes from alkynes

Table 2.7. Formation of heteroatom-substituted carbene complexes from alkynes, vinylidene complexes, and alkynyl complexes.

Highly reactive organic vinylidene and allenylidene species can be stabilized upon coordination to a metal center [1]. In 1979, Bruce et al. [2] reported the first ruthenium vinylidene complex from phenylacetylene and [RuCpCl(PPh3)2] in the presence of NH4PF6. Following this report, various mthenium vinylidene complexes have been isolated and their physical and chemical properties have been extensively elucidated [3]. As the a-carbon of ruthenium vinylidenes and the a and y-carbon of ruthenium allenylidenes are electrophilic in nature [4], the direct formation of ruthenium vinylidene and ruthenium allenylidene species, respectively, from terminal alkynes and propargylic alcohols provides easy access to numerous catalytic reactions since nucleophilic addition at these carbons is a viable route for new catalysis (Scheme 6.1). 

Scheme 8.74, path B is reminiscent of the electrophilic attack at oxygen of acylmetal complex shown in Eq. 8.5. Another electrophilic route to carbene complex is the reaction of alkylmetals with Ph3C" , as shown in Eq. 8.25 [136]. An electrophilic attack that is similar to Scheme 8.74, path B but appears potentially more signihcant in catalysis is that of alkynylmetal complex to generate vinyli-dene ligand. Although Scheme 8.17 described direct formation of the vinylidene complex from M+ and terminal alkyne, this complex is sometimes derived by treatment of M-C = CR with H+ via -attack [137]. 

SCHEME 21.52 Formation of transition metal-vinylidene complexes from terminal alkynes. 

Based on the well-known ability of Rh to form vinylidene complexes from terminal alkynes [26, 27], Trost and Rhee tested a series of Rh(I) phosphine complexes as catalysts for the cycloisomerization of homo- and bis-homopropargylic alcohols to obtain dihydrofurans and dihydropyrans under neutral conditions [28] (Scheme 9). 

The dominant factors reversing the conventional ds-hydroboration to the trans-hydroboration are the use of alkyne in excess of catecholborane or pinacolborane and the presence of more than 1 equiv. of EtsN. The P-hydrogen in the ris-product unexpectedly does not derive from the borane reagents because a deuterium label at the terminal carbon selectively migrates to the P-carbon (Scheme 1-5). A vinylidene complex (17) [45] generated by the oxidative addition of the terminal C-H bond to the catalyst is proposed as a key intermediate of the formal trans-hydroboration. 

Already 20 years ago, Antonova et al. proposed a different mechanism, with a more active role of the transition metal fragment [3], The tautomerization takes place via an alkynyl(hydrido) metal intermediate, formed by oxidative addition of a coordinated terminal alkyne. Subsequent 1,3-shift of the hydride ligand from the metal to the P-carbon of the alkynyl gives the vinylidene complex (Figure 2, pathway b). 

The acetylene coordinates trans to the least o electron donor group, chlorine. Coordination of the C-H bond is a less favorable alternative to coordination of the n system. The o C-H complex is 17.1 kcal.mol 1 less stable than the rc-alkyne complex (Figure 5). From this c C-H intermediate the 1,2 shift is possible with a relatively small activation barrier (+15.5 kcaLmol 1) to yield the vinylidene complex. However this mechanism is in contradiction with the labeling experiment. 

Acyl complexes can also result from the reaction of terminal alkynes with cationic, hydrated complexes of iron (Entry 4, Table 2.7) [47]. An electrophilic vinylidene complex is probably formed as intermediate this then reacts with water and tautomerizes to the acyl complex. 

From Vinylidene Complexes Generated from Alkynes... 

Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding 7t-complexes. With terminal alkynes the corresponding 7t-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [128,130,132,133,547,556-569]. With mononuclear rj -alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-Fl bond to yield a hydrido alkynyl eomplex, followed by 1,3-hydrogen shift from the metal to Cn [570,571], or (b) eoneerted formation of the M-C bond and 1,2-shift of H to Cp [572]. 

Table 1.1 Some metal-vinylidene complexes, L M=C=CRR, prepared from 1-alkynes.

Formation of metal acyl or carbonyl complexes from 1-alkynes in the presence of water is often assumed to proceed via attack on an intermediate vinylidene complex to give a hydroxycarbene complex (Equation 1.24) ... 

Furthermore, Fischer rendered this chemistry more practical by generating vinylidene complexes of pentacarbonylchromium and tungsten directly in situ from terminal alkynes [9]. For example, treatment ofterminal alkynes with WCO)5(CH2Cl2), generated by photolysis of W(CO)6 in CH2CI2, gave thermo-labile tt-alkyne W(CO)5... 

Scheme 5.6 Generation of Generation of vinylidene complexes directly from alkynes.

As described in this chapter, vinylidene complexes of Group 6 metals have been utilized for the preparation of various synthetically useful compounds through electrophilic activation or electrocyclization of terminal alkyne derivatives. These intermediates are quite easily generated from terminal alkynes and M(CO)6, mostly by photo-irradiation and will have abundant possibilities for the catalytic activation of terminal alkynes. Furthermore, it should be emphasized that one of the most notable characteristic features of the vinylidene complexes of Group 6 metals is their dynamic equilibrium with the it-alkyne complex. Control of such an equilibrium would bring about new possibilities for unique metal catalysis in synthetic reactions. 

Ruthenium vinylidene species can be transformed into small carbocyclic rings via carbocyclization reactions. Ruthenium vinylidene complex 2, generated from the electrophilic reaction of alkyne complex 1 with haloalkanes, was deprotonated with "BU4NOH to give the unprecedented neutral cyclopropenyl complex 3 (Scheme 6.2) [5]. Gimeno and Bassetti prepared ruthenium vinylidene species 4a and 4b bearing a pendent vinyl group when these complexes were heated in chloroform for a brief period, cyclobutylidene products 5a and Sb formed via a [2 + 2] cycloaddition between the vinylidene Ca=Cp bond and olefin (Scheme 6.3) [6]. 

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

Most pioneering studies of vinylidene-mediated catalysis were concerned with transition metals from Groups 6 and 8 of the periodic table (Chapters 5, 6, 10). These endeavors led to the recognition that unique and valuable products can result from the modification of alkynes via their vinylidene complexes. Examples of Group 9-11 transition metal vinylidene-mediated catalysis have increased in number recently. Reported examples share some common features. 

The ability to harness alkynes as effective precursors of reactive metal vinylidenes in catalysis depends on rapid alkyne-to-vinylidene interconversion [1]. This process has been studied experimentally and computationally for [MC1(PR3)2] (M = Rh, Ir, Scheme 9.1) [2]. Starting from the 7t-alkyne complex 1, oxidative addition is proposed to give a transient hydridoacetylide complex (3) vhich can undergo intramolecular 1,3-H-shift to provide a vinylidene complex (S). Main-group atoms presumably migrate via a similar mechanism. For iridium, intermediates of type 3 have been directly observed [3]. Section 9.3 describes the use of an alternate alkylative approach for the formation of rhodium vinylidene intermediates bearing two carbon-substituents (alkenylidenes). 

The proposed reaction mechanism is shown in Scheme 9.15. Starting from the phenyl-rhodium complex 87, alkyne rearrangement is expected to furnish the phenyl-vinylidene complex 88. Migration of a phenyl ligand onto the vinylidene moiety of 88 must occur such that the vinyl Rh-C bond and the enone tether of the resultant complex (89) attain a cis-relationship to one another. Intramolecular conjugate... 

Vinylidene complexes have been obtained from reactions between PhC=CEPh3 (E = Si, Ge, or Sn) and manganese complexes (11, 18) solvolysis of the C-E bond is followed by transfer of a proton from the solvent. The yields (E = Si, 0% Ge, 1% Sn, 15%) are inversely proportional to the stability of the intermediate tp-alkyne complex. 

Protonation or alkylation of several ethynyl-metal derivatives gives the corresponding vinylidene complexes in high yield (14,24). This is a convenient route to the disubstituted vinylidene complexes, as well as the parent compounds, which cannot be obtained from l-alkynes they are formed if MeOS02F or [R30]+ (R = Me, Et) is used as alkylating agent ... 

Although no simple vinylidene complex was obtained from reactions between Cr(CO)5(OEt2) and methyl propiolate, complexes 60, 61, and 62 were formed in the ratio 4 5 1 (22) the latter two each contain two molecules of the alkyne, and may be formed from an intermediate Cr[=C=CH(C02Me)](C0)5 complex (22). 

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